Ultra-high-frequency signaltranslating stage



Aug. 13, 1940.

R. L. FREEMAN uLTRA-Hmn-FREQUENCY SIGNAL-TRANSLATING STAGE Filed Jan. 12, 1939 Negative Grid Bias RBERT L FREE N ATTORNEY Patented Aug. 13,1*940 l UNITEDA STATES arribaron-FREQUENCY SIGNAL- 'raANsLA'rmc suor;

Robert L. Freeman, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delay 'Ware .Application January l2, 1939, Serial No. 250,502

11 claims.

'l 'I'his invention relates generally to ultra-highfrequency signal-translating stages for operation at such high frequencies thatthe conductance lof the input circuit of the stage is an appreciable factor in determining the response.

characteristic of the stage.

The input'conductance of conventional vacu-I um tubes utilized in ultra-high-frequency translating stages, such as a stage of ultra-high-frequency ampliiication, materially reduces the response of the stage. At frequencies aboveten megacycles, or thereabout, the input conductance of a conventional vacuum tube is appreciable and at frequencies higher thanv fifty mega- 'cycles the input conductance, rather than the inherent tube and circuit capacitance, becomes the limiting factor in the response ofthe stage. The reason for this condition is that themaximum impedance which can be developed across go the input circuit of the stage at such ultra-high frequencies is limited by the input conductance of the tube. 4 v

Also, at ultra-high frequencies, the input capacitance of sucha stage and the variation of input capacitance with'variations of' the trans- 1 conductance of the tube are both appreciable,

adding other limitations to the response characteristic of the system. While there have been proposed arrangements for reducing the input capacitance and for maintaining it substantially-uniform with variations of transconductance of the tube of such an ultra-high-frequency stage, such arrangements in themselves' have not been eiective to compensate for the conductance of the input circuit and some arrangements for stabilizing the input capacitance have even introduced appreciable additional conductance into the input circuit. It is, therefore, desirable to provide-an arrangement which may be o used with such systems for maintaining relatively constant-input capacitance and which is effective also to reduce to a low value the conductance of the input circuit of the stage.

It is an object of the present invention, therefore, to provide an improved lulti'a-l'iiglr-frequency signal-translating stage which is not subject to the above-mentioneddisadvantages ofv arrangements of the prior art.

It is a further object ofthe invention to prow* vide animproved ultra-high-frequency signaltranslating stage, the response of which is not substantially limited by theinput conductance oi' the vacuum tube included in the stage.

It is also an object of the in ventionA to pro- 55 vide an ultra-high-frequency signal-translating stage, including means for stabilizing or main-,

taining relatively constant the input capacitance -of the stage, the response of the stage being not substantially limited by the input conductance oi!l l .g the vacuum tube included in the stage.

(Cl. P19- 171) In accordance with the invention, there is provided an ultra-high-frequency signal-translatingl stage comprising a vacuum tube of the screen-- grid type having input, output, and screenv circuits.

to a common terminal, such as ground. 'I'he input circuit oi the vacuum tube comprises a cathode, a cathode lead having an external terminal, and a control electrode having an external terl0 minal, the cathode and the control electrode of the .vacuum tube having capacitance therebetween, and the cathode terminal and the controlelectrode terminal having a positive conductance therebetween. Capacitance means are provided 16 between the control-electrode terminal and the common terminal, together with a connection including inductance between the cathode and the common terminal. The reactive constants of the capacitance means and the inductance are so 80 proportioned that the voltage developed across the inductance and coupled to the control electrode through the capacitance means eifectively r'eects a negative conductance between the cathode and control-electrode terminals which 25 neutralizes a substantial portion of the abovementioned positive conductance.

In other embodiments of the invention there are pro'vided means for stabilizing .theinput capacitance of the stage, due to variations in trans- 30 conductance of the tube utilized, together with means for eliminating at least a substantial portion of the conductance of the input circuit, which vis due to either or both the transit time oi' electrons through the tube and the inherent in- 35 vention itself. however, both as to its organizaytion and method of operation, together with other and further. advantages thereof, will best be understood with reference to the following speciviication taken in connection with the accompanying drawing in which Fig. l is a circuit di agramof a stage of amplification incorporating the invention; Fig. 2 isla graph of certain operating characteristics of thecircuit of Fig. 1; 5g'

while Figs. 3 and 4 are circuit diagrams of modiiications of the invention utilized in conjunction with diii'erent input-capacitance stabilizing arrangements. l

The input conductance of conventional vacuum tubes is determined by three factors: (1) the dielectric losses ci the tube insulatorsand the tube base; (2) feedback from the anode-cathode circuit to the grid-cathode circuit through the 'grid-cathode capacitance oi the tube and the o` The screen circuit, and in some cases the i l output circuit, is returned for signal frequencies self-inductance of the cathode lead, the latter being commonto the input and output circuits;

and (3) the transit time of electrons through the component of the input conductance dependent on the transit time of electrons through the tube.

The circuit of Fig. 1 illustrates the general principles of the invention which may be utilized to compensate for the two last-mentioned components of input conductance.- Referring now to Fig. 1, there is illustrated an ultra-high-frequency signal-translating stage including a vacuum-tube I0, input terminals II, I2, and output terminals I3, I4. The grid-cathode capacitance of tube I0 is represented by condenser I5, shown in dotted lines for the reason that it is usually comprised entirelyof inherent interelectrode capacitance. The inductance of the cathode lead of .tube I0 is represented by inductance I6, also shown in dotted lines for the reason that it represents inherent inductance, the simulated inductance due to transit` time effects, or both, in the cathode lead. The impedance of the output circuit is represented schematically by load circuit II connected across output terminals I3, I4. The signal input, applied at input terminals lI, I2, is impressed upon the input electrodes of tube I0 through inductively coupled inductances -4 I8 and IS. Inductance 20 is connected between the external terminal of the cathode lead and a common terminal, such as ground, which terminal is coupled to the external terminal of the control electrode of the tube through a condenser 2I effectively to eliminate the conductance introduced across the input circuit of tube I0 by the feed-back circuit comprising inductance I6 and capacitance i5. It will be understood that inductance I6 may represent either or both of the actual inherent inductance of the cathode lead of tube Ii) and the simulated inductance equivalent in effect to that of the transit time of electrons through the tube I, and that condenser 2| is, in the usual ultra-high-frequency stage, comprised in whole or in part of the inherent capacitance of the input circuit. screen and anode circuits of tube I are returned for signal frequencies to the common terminal through by-pass condensers 22 and 23, respectively.

In considering the operation of the circuit just described, it' will irst be assumed that the input conductance due to transit time eiects are negligible and that the input conductance of the system is caused only by feedback through inductance I5, which under the conditions assumed represents the actual inherent inductance of the cathode lead of tube I0, common to both its input and output circuits, and condenser I5. In the following analysis the symbols I, L, and C, together with subscripts corresponding to the reference numerals of the circuit elements of Fig. 1, identify, respectively, the current, inductance, and capacitance associated with the parcuitof tube I0 tothe right'of dotted line A"of Fig. 1 and assume a voltage E developed across inductance I8. Also, let

Under the conditions of Equation 9 from which Equation 10 was derived, the admittance Y of the input circuit to the right of dotted line A is independent of the transconductance of tube I and furthermore is a pure susceptance.

From Equation 9 it is seen that ,Lm and aLl, and L=Lg-;

It is thus seen that a circuit proportioned in accordance with the above expressions is effective to provide zero input conductance for tube IB at any frequency well below the resonant frequency of the feed-back circuit elements and for any value of transconductance of tube I0.

Characteristic curves for the circuit of Fig. 1 are shown in Fig. 2. The normal operating characteristic of the tube utilized in a circuit not incorporating the' invention is shown by curve B of Fig. 2. AThe characteristic of the circuit of Fig. 1 proportioned in accordance with the above principles is shown in curve C. The residual input conductance which' is not eliminated is that due to dielectric losses, mentioned above. The system of Fig. 1 may also be proportioned to reduce the input conductance to zero for any particular'value of transconductance by providing additional feed-backv energy at the selected transconductance over that necessary .to neutralize only the etlect of inductance in the cathode lead, the characteristic curve for this arrangement being shown in curve D.'

v 5 The foregoing analysis of the operation ot Fig.

1 was based upon the assumption that conductance dueto transit time eilects was negligible v and that only the feedback through inherent in- `ductance Il andcapacitance Il effected the in- 10 put conductance of the system. The magnitude of the component ofinput conductance of a conventional vacuum tube which is dependent on the transit time oi' electrons through the tube has been derived mathematically by D. O.

l5 North see "Proceedings of.. the Institute of Ra- 'dio Engineers, vol. 24 No. 1, January, 1936, page 108 et seq.). A mechanical explanation of the eiect of transit time of yelectrons on the input conductance oi' a vacuum tube has been developed zo by W.R. Ferris (see Proceedings of the Institute of Radio Engineers, vol. 24, No. 1, January. 1936, pge 82, et seq.).

3g,y where Gx. is the component of input conductance due to inductance o! the cathode lead.

It is thus seenthatGmaybe expremedas G=1lzC|5Lf1Qn where L'm is a ilctitious cathode lead inductance 40 that woum account for the mm1 input conductance of the tube, having a .value 45. Therefore, for measured values of the left- Y hand member of Equation 14, there can' be calculated the correct value of Lu for use in Equation 12 to obtain neutralization ofthe input conductance of the tube. This value will give exact 5 compensation for any lfrequency but i'or only one value of transcnductance oi the tube. due to the fact that F does vary slightly with transconductance. However. a moderate'degree oi' neutralization is provided over a reasonable range 5@ of variation of transconductance of tube il.

From Equation 1li there may be derived the equation:

recuuo b ` If. in Equation l2. L'n is substituted for Lu.

L'xeC'xsLsoCn (17) Therefore, from Equations 1B and 1'? ,LaoCnd-an (18) That is to say, when both the screen and output circuits are returned for signal'irequencies to a common terminal. such as ground, the means for 4neutralizing the positive conductance between the v cathode terminal 4and control-electrode terminal of the vacuum tube Il comprises the connection including inductance Il between the cathode and 15 common terminal and capacitance .means Il 'Ihese authors havev shown that the component of input conductance between the terminal of the control electrode and the common terminal for a given value of 'anguiar frequency w and transconductance mi. of the vacuum tube III, the product of inductance 2l and'capacitance 2i being made equal to the g only in that a cathode resistor 24 has been added. 10

Similar circuit elements have been given identical reference numerals in the two ilgures. Such a cathode resistor has beenutilized in prior art 1 arrangements to stabilize' the input .capacitance of a vacuum tube. Since the correct value of re- Il sistor 24 to provide stabilization for input capacitance of the stage of Fig. 3 does notcompletely eliminate input conductance, inductance 2|, which must be proportioned in accordance with the principles `outlined above; is also` provided. D The circuit is then eie'ctive to reduce the conductance of the input circuit of the stage to a very low value at high values of transconductance andv to stabilize the input capacitance over the entire range of transconductance variation. Il

The circuit of Fig. 4 is also essentially similar to that of Fig. 1 and similar circuit elements have been given identical reference numerals. The circuito! Fig. 4 diiers from that oi' Fig. lin that provision is made for applying a different & control bias to theV control electrode and the suppressor-grid electrode of vacuum tube il. I he ratio of the applied biases is such as to vary the transconductance of .vacuum tube II over a desired range and to maintain' the input capacitance of the system substantially constant over the range of variation of transconductance of vacuum tube I0, in accordance with the prinv ciples outlinedin detail in the copending application o: Johh F. Farrington, Serial No. I 165.611, tiled September 25, 1937. While the screen grid `oi! vacuum tube I0 of Fig. 4 is bypassed to the cathode by condenser 23 as in the circuit of Fig. 1, the by-pass condenser I2 for the anode circuit`may be coupled eith'er to the common terminal between inductances II and 2l through a switch 22' as shown, or to a tap on inductance 20, by operation of the switch 22' to its lower position.

In considering the operation oi the circuit oi' I `appreciably as the biases of tube i0 are increased.

In order to compensate for-such a variation or the conductance of the input circuit, it is, therefore, necessary to provide a current through inductance 20, the effect oi which is complementary to that ofthe eiective inductance I6 over the range of variation of the biases of tube l0. 'lhe screen-grid current, under the dual-bias variation, has the desired characteristic. and the cir- .cuit constants may, therefore, be proportioned.' in accordance with the principles outlined above,

to provide a relatively constant conductance in n the input circuitof vacuum tube I0 over the range of .grid-anode, transcondnctance while simultaneouslyv maintaining the input capacitance relatively constant. .l

With some tubes, the controlelectrode screenu electrode transconductance increases more rapidly than the input conductance with increasing bias voltages. Therefore, in order to provide the desired complementary characteristic, the anode current may be caused to flow through a portion of inductance 20. This circuit arrangement is provided by the'operation of switch 22' to its lower position.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in` the art that various changes and modiiications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as-fall within the true spirit and scope of the invention.

What is claimed is:

1. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having inputoutput, and screen circuits, said screen circuit being returned for signal frequencies to a common terminal, said input circuitcomprising a cathode, a cathode lead having an external terminal, and a control electrode having an external terminal, said cathode and control electrode having capacitance therebetween and said cathode terminaland said control-electrode terminal having a positive conductance therebetween, a condenser connected 'between the terminal of said control electrode and said common terminal, and an added inductance connected between said cathode terminal pled to said control-electrode terminal through said condenser effectively reflects a negative conductance between said cathode and controlelectrode terminals which neutralizes a substantial portion of said positive conductance.

2. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having input, output, and screen circuits, said screen and output circuits being returned for signal frequencies to a common terminal, said input circuit comprising a cathode, a cathode lead having an external terminal, and a control electrode having an external terminal, an added inductance element connected between the external terminal of said cathode and said common terminal, and a condenser connected between the external terminal of said control electrode and said common terminal, said cathode and control electrode having capacitance therebetween, said cathode terminal and controlelectrode terminal having a positive conductance therebetween which may be substantially simulated by an inductance in said cathode lead, the ratio of the capacitance of said condenser to the capacitance between said control electrode and cathode being equal substantially to the ratio of the simulated inductance to the` inductance of said added inductance element.

3. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having input, output, and 'screen circuits, said screen and output-circuits being returned for signal frequencies to a common terminal, said input circuit comprisinga cathode, a

pacitance therebetween and said cathode lead comprising appreciable inherent inductance, said capacitance and said inductance being eiective to produce positive conductance between saidv control-electrode and cathode terminals duringnormal operation of said stage, a condenser connected between said control-electrode terminal and said common terminal, and an added inductance connected between said cathode terminal and said common terminal, the reactive constants of said condenser and said added inductance being so proportioned with respect to those of said inherent inductance and said capacitance that the voltage developed across said added inductance and coupled to said control-electrode terminal through said condenser effectively reects a negative conductance between said cathode and control-electrode terminals which neutralizes a substantial portion of said positive conductance.

4. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having input, output, and screen circuits, 'said screen and output circuits being returned for signal frequencies to a common terminal, said input circuit comprising a cathode, a cathode lead having an external terminal, and

a control electrode having an external terminal,

said cathode and control-electrode terminals having a positive conductance therebetween due to transit time of electrons during normal operation-of said stage, a condenser connected between said control-electrode terminal and said common terminal, and an added inductance connected between said cathode terminal and said common terminal, the reactive constants oi said condenser and said added inductance being so proportioned that the voltage developed across said added inductance and coupled to said control-electrode terminal through said condenser effectively reects a negative conductance between said cathode and control-electrode terminals which neutralizes a substantial portion of said vpositive conductance.

5. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having input, output, and screen circuits, said screen and output circuits being returned for signal frequencies to a common terminal, said inputicircuit comprising a cathode, a cathode lead having an external terminal, and a control electrode having an external terminal, said cathode and control electrode having capacitance therebetween and said cathode terminal and said control-electrode terminal having' therebetween a positive conductance which may be simulated by an inductance in said cathode lead during normal operation of said stage, a condenser connected between said control-electrode terminal and said common terminal, said condenser being effective with said capacitance to tune said input circuit within the range of operating frequencies of said stage, and an added inductance connected between said cathode terminal and said common terminal, the reactive constants of said condenser and said added inductance beingl so proportioned with respect to those of said simulated inductance and said capacitance that the voltage developed across said added inductance and coupled to said control-electrode terminal through said condenser effectively reilects a negative conductance between said cathode and control-electrode terminals which neutralizes asubstantial portion of said positive conductance.

6. An ultra-high-frequency signal-translating stage comprising a vacuum tubeof the screengrid type operable over a given range or transconductance and having input, output, and screen circuits, said screen circuit being returned for signal frequencies to a ,common terminal,

saidvacuum-tube input .circuit comprising a.

cathode, acathode lead having an external terminal, and a control electrode having an external' terminal, said cathode and control electrode havinga capacitance therebetween and said cathode terminal and said control-electrode terminal tralizes asubstantial portion of said positive conductance.

'7. An uitra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type operable over a given range of transconductance and having input, output, andscreen circuitsfsaid screen and output circuits being returned for signal-frequencies to a common terminal, said inputl circuit comprising a cathode, a cathode lead having an external terminal, and a control electrode having an ex, ternal terminal, said cathode and control electrode having capacitanceA therebetween and said cathode vterminal and said control-electrode terminal having therebetween a positive conductance, a condenser connected between said control-electrode terminal and said common terminal, and means including an added inductance and a resistor connected between said cathode terminal and said4 common terminal, said resistor being proportioned to maintain said capacitance substantially uniform over said range of transconductance and the reactive constants of said condenser and said added'induc'tance being so proportioned that the voltage developed across said added inductance and coupled to said 'control-electrode terminal through said condenser effectively reiiects a negative conductance electrode and said additional electrode relatively g to vary the transconductance of said tube and 7&54

between said cathode and control-electrode' terminals which neutralizes a substantial portion of said positive conductance.

8; An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screenvgrid type having input, output, and screen circuits, said screen circuit being returned for signal frequenciesA vacuum-tube input circuit comprising a cathode, a cathode lead having an external terminal, and

-a control electrode having an external terminal, A an ad tional electrode in said vacuum tube, said cathoglV therebetween and said cathode terminal and said and control electrode having capacitance control-electrode terminal having therebetween a positive conductance. means Afor applying simultaneously adjustable. biases to said control proportioned l maintain said capacitance substantially constantv over the. range of transconductance variation.' acondenser connected beto a common terminal, said tween said control-electrode'terminal and said common terminal, and means including an added inductance connected between said cathode terminal and said common terminal, the reactive constants of said condenser and4 said added inductance being so proportioned that the voltage developed across said added inductance and coupled to said control electrode through said condenser effectively reflects a negative conductance .between said cathode and control-electrode terminals which neutralizes a substantial portion 'of saidl vpositive conductance. f

9. An ultra-high-frequency signal-translating stage comprising a, vacuum tube of the screengrid type having input, output, and screen circuits, said screen circuit being returned for signal frequencies'to a common terminal, said input circuit comprising a cathode, a cathode lead having an external terminal, and a control electrode having an external terminal, said output circuit comprising an anode, a screen electrode in said vacuum tube, said cathodel and control electrode having capacitance therebetween and said cathode terminal and said control-electrode terminal 'having a positive conductance therebetween, means for applying simultaneouslyadjustable biases to said controlv electrode and an additional electrode in said tube relatively proportloned to vary the transconductance of said .tube while maintaining said capacitance substantially constanty over the range of variation of said transconductance, a condenser connected between said control electrode terminal and said common terminal,- and an added inductance connected between said cathode vterminal and said common terminal. and including an intermediate terminal connected to said anode, said added inductance being coupled between said cathode terminal and Said control-electrode terminal vthrough said condenser, the reactive constants of said condenser and said addedinductance being so proportioned that the voltage developed across said added inductance and coupled to said control electrode through said condenser eiectively reilects anegative conductance between said cathode and control-electrode terminals which neutralizes a substantial portion of -said positive conductance.

10. An ultra-high-frequency signal-translating stage comprising a vacuum tube of the screengrid type having input, output, and screen cirv cuits, said screen circuit being returned for signal frequencies to a common terminal, said input circuit comprising a cathode, a cathode lead having an external terminal and a control electrode having an external terminal, said cathode and control electrode having capacitance therebetween and said cathode terminal andsaid control-electrode terminal having positive conductl ance therebetween, capacitance means between said control-electrode terminal and saidcommon terminal, and a connection including inductance between said cathode and said common terminal, the reactive constants oi said capacitance means and said inductance. being so proportioned that the voltage developed across said inductance and coupled' to said control electrode through said capacitance means effectively reflects a negative conductance between said cathode and controlelectrode terminals which neutralizes a substantial portion of said positive conductance.

1l. An ultrahigh-frequency signal-translating stage comprising a vacuum tube of the screen- `grid type having input, output, and screen circuits, said screen'and output circuits being returned for signalfrequencies to a common terminal, s'aid input circuit comprising a cathode, a

cathode lead having an external terminal and a control electrode having an external terminal, said cathode terminal and control-electrode terminal having a predetermined positive conductance therebetween at a given value of angular frequency and transconductance of said vacuum tube, and.- means for neutralizing said 10 positive conductance comprising a connection including inductance between said cathode and said common terminal, and capacitance means between the terminal of said control electrode' ROBERT L. FREEMAN. 

